Contact lenses are generally used to correct various types of vision problems. See, for example, U.S. Pat. Nos. 4,573,774; 5,650,837; and 5,652,638, which are incorporated herein in their entireties by reference. Also, various texts discuss the principles of contact lens design and manufacturing: Ruben et al., Contact Lens Practice (Chapman & Hall, London); Stein, Slatt & Stein, Opthalmic Terminology (C.V. Mosby Company, St. Louis, 1987), which are also incorporated herein by reference.
Astigmatism occurs when a portion of the eye focuses light at a different focal point than the rest of the eye, i.e., it is a refractive error of the eye in which the rays of light do not come to a single point focus on the retina. Toric lenses, or lenses with cylinder power, typically are used to correct for astigmatism. For toric lenses, the cylinder power is along the cylinder radius, which is orthogonal to the cylinder axis. The cylinder power of the toric lens results in the eye focusing light rays at a common focal point.
As indicated above, toricity is used to correct astigmatism so that it focuses light rays at a common point. However, the common focal point may be located at an incorrect position with respect to the retina. This condition is referred to as either "myopia" or "hyperopia". Myopia which typically results from a steep cornea that causes light rays to focus at a focal point which is short of the retina. Conversely, hyperopia is a case in which a flat cornea causes light rays to focus at a focal point that is behind the retina. Sphere power is included in a lens to cause light to property focus on the retina.
Contact lenses may include either or both spherical correction or cylinder correction depending on the particular circumstances of the lens wearer's eyes. Lenses that have only spherical correction are spherically symmetric and, thus, any rotation of the lens inside the eye is irrelevant and does not disturb the intended vision correction. Toric contact lenses, on the other hand are typically designed to include areas of different thickness, resulting in a spherically asymmetric lens. As a result, toric contact lenses need to be rotationally stabilized in the eye to insure that the lens is properly oriented with respect to the eye in order to carry out its function of correcting the vision of particular areas of the eye.
Rotational stability may be achieved using a number of designs including inferior truncation, double truncation, thin zones (also called "double slab-off"), back surface toricity and prism wedge profile. These rotation stabilizing designs may be used individually or in combination. One common feature of these rotation stabilizing designs is the use of different thickness areas in the lens to achieve rotational stability. For example, in the case of a "slab off" design, the top and bottom portions of the lens are thinned out such that when they are positioned under the eyelids they are held in place by the lids. At the same time, the thicker portions of the lens are positioned in between the eyelids where they too are held in place by abutting against the eyelids.
The process for manufacturing differential thickness lenses typically includes a curing step, which may involve UV, heat, or other similar curing process. However, as a result of the curing step, the material of the lens shrinks at different rates due to the differential thicknesses. This differential shrinkage causes the cylinder power to change as a function of position. The sphere power is also caused to change as a function of position. Conventional approaches compensate for this error using a single base curve which is applied to all lenses regardless of the angular position of the cylinder power and sphere power.